1. Field of the Invention
The present invention relates to a document reading apparatus that reads a document and processes a document image of a read document. Further, the present invention relates to an image processing method of processing a document image of a read document.
2. Description of the Related Art
A document reading apparatus comprises a document reading portion including a document table having a document placement surface, a light source for exposing a document placed on the document placement surface to light, an imaging element for receiving light from the document exposed by the light source, and an imaging optical system for focusing the light from the document onto the imaging element to thereby form an image thereon. The imaging optical system has an imaging lens and a group of mirrors which lead the light from the document to the imaging lens. In the documents reading apparatus, by imaging the light from the document placed on the document placement surface sequentially line by line along a main scanning direction of the document on the imaging element while moving the group of mirrors in a sub-scanning direction relative to the document table, the document can be read.
In the document reading apparatus as described above, the group of mirrors is made to move for reading the document placed on the document placement surface and therefore, it is structurally difficult to hold the entire part of the document in focus at the same time. Focusing on a part of the document causes the remaining part of the document to be out of focus.
The focus adjustment at a focus adjustment step in manufacturing the document reading apparatus is achieved by focusing on the document at a document-reading start position. In this case, there will be seen a gradual increase in loss of focus on the document from the document-reading start position toward a document-reading end position. In consideration of this point, the focus adjustment may be achieved by shifting the focal spot at the document-reading start position in the reverse direction in anticipation of loss of focus at the document-reading end position. In this case, focusing on the document can be attained only at an intermediate position between the document-reading start position and the document-reading end position.
As described above, in the document reading apparatus of conventional design, it is structurally difficult to focus on the entire part of the document, thus causing a problem that the quality of the document image of the read document is hard to be equalized overall.
Japanese Unexamined Patent Publication JP-A 10-319305 (1998) discloses a technique that a lens carriage on which a lens and an image sensor are mounted, is made to move pitch by pitch so that black stripes and white background on a focus adjustment plate are read, to thereby obtain a position of the lens carriage exhibiting the maximum contrast, at which position the lens carriage is fixed. The focus adjustment plate is attached to a front portion at a document-reading start position. The technique just described is not able to solve the above-described problem because, in the technique, the focus adjustment is carried out only one place in the sub-scanning direction.
Further, in the case where the document image is a color image, it is difficult to eliminate a color shift from the entire document image. The color shift means that positions of respective colors in the document image are shifted relatively from each other. Such a color shift can be corrected to some extent, by adoption of achromatic setting in which a glass dispersion property is utilized, or by interposition of a diffractive-optical element between the document and the imaging element. However, these solutions will cause troubles such as an increase in production cost, a decrease in light transmission, and an increase in size of the apparatus, and moreover, it is difficult to compensate the temperature.